In run-webkit-tests, start xvfb if necessary.

third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/port/linux_unittest.py

Index: third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/port/linux_unittest.py
diff --git a/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/port/linux_unittest.py b/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/port/linux_unittest.py
index fe363f73c44d767964611998b915a8c11137a37d..9b6a25e55f438b7eb52381d4a3d731e2a2cf9052 100644
--- a/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/port/linux_unittest.py
+++ b/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/port/linux_unittest.py
@@ -114,3 +114,33 @@ class LinuxPortTest(port_testcase.PortTestCase):
         port.clean_up_test_run()
         self.assertEqual(port.host.environ['HOME'], '/home/user')
         self.assertFalse(port.host.filesystem.exists(temp_home_dir))
+
+    def test_setup_test_run_starts_xvfb(self):
+        port = self.make_port()
+        port.host.executive = MockExecutive(exit_code=1)
+        port.setup_test_run()
+        self.assertEqual(
+            port.host.executive.calls,
+            [
+                ['xdpyinfo', '-display', ':99'],
+                ['Xvfb', ':99', '-screen', '0', '1280x800x24', '-ac', '-dpi', '96'],
+            ])
+        env = port.setup_environ_for_server()
+        self.assertEqual(env['DISPLAY'], ':99')
+
+    def test_setup_test_runs_finds_free_display(self):
+        port = self.make_port()
+        port.host.executive = MockExecutive(
+            run_command_fn=lambda args: 1 if ':102' in args else 0)
+        port.setup_test_run()
+        self.assertEqual(
+            port.host.executive.calls,
+            [
+                ['xdpyinfo', '-display', ':99'],
+                ['xdpyinfo', '-display', ':100'],
+                ['xdpyinfo', '-display', ':101'],
+                ['xdpyinfo', '-display', ':102'],
+                ['Xvfb', ':102', '-screen', '0', '1280x800x24', '-ac', '-dpi', '96'],
+            ])
+        env = port.setup_environ_for_server()
+        self.assertEqual(env['DISPLAY'], ':102')